1. Field of the Invention
This invention relates generally to blowout prevention equipment as used, for example, in the drilling of oil and gas wells. More particularly, this invention relates to blowout prevention equipment of the type including a ram assembly that will shear a pipe string extending through the blowout preventer when the rams of the assembly are forced together. Such ram-shears are desirable in blowout prevention equipment used in a well operation where there is the possibility of a situation arising such that it would be desirable to sever the drill pipe or tubing. This situation might arise, for example, during a drilling operation conducted from a floating vessel if the anchor system should fail or for some other reason the vessel could not be safely retained over the well site. In such situations, it is sometimes necessary to shear the pipe string at the wellhead to avoid severe damage to the wellhead or the floating vessel. In some instances, such as when the pipe string is being removed from the well to change drilling bits, and an emergency arises before the pipe string is completely removed from the well, the sheared pipe string may fall back into the well causing a delay in operations while the severed pipe string is recovered, and in some cases the well is irretrievably damaged such that it is a total loss. Thus, there has been a need for a blowout preventer which enables an operator to sever a pipe string and at the same time prevent the severed pipe from falling down into the well.
2. Description of the Prior Art
Ram-shear assemblies for blowout preventers are now routinely available. Conventional devices for shearing a pipe string and shutting off a well are effective, but prior to this invention ram-shear assemblies have had no provision for retaining the severed pipe string and preventing it from falling down into the well. A ram-shear assembly of the type presently used commercially is described in U.S. Pat. No. 3,561,526, wherein the shearing assembly also provides a seal after the pipe has been severed. Blowout prevention equipment including a ram-shear assembly and a shutoff ram assembly below the shearing assembly is described in U.S. Pat. No. 3,736,982. The lower assembly in the apparatus described in that patent provides a pressure seal about the severed lower portion of the pipe string, but it has no provision for preventing the severed portion of the pipe string from falling down into the well, such that it is primarily useful in the situation where the drill bit is resting on the bottom of the hole.
A device for cutting a tubing string extending into a well and clamping means for retaining the severed pipe is described in U.S. Pat. No. 3,692,107. This device is, however, not suitable for blowout prevention utilization, and is not so intended.
Thus, prior to this invention there has been no suitable equipment available to prevent a sheared pipe string which is suspended from drilling or servicing equipment from droping into the well when ram-shear blowout prevention devices are actuated in a threatened or actual blowout crisis. Without means for suspending the severed pipe string, the pipe string will drop to the bottom of the well, causing delay, expense and possible loss of the well.